Conventional cameras use lens systems comprised of substantially spherical lenses. General design principles are applied to reduce aberrations and to achieve a desired image quality. When less aberration is desired, more lens elements are added. Examples of general structural design principles used in conventional camera lenses include symmetry around the system aperture, combining large diameter lenses with small diameter lenses, combining low dispersion (Abbe number>50) and high dispersion lenses (Abbe number<50), use of substantially spherical lenses, use of additional aspherical lenses, use of thin and strongly bent lenses and use of artificial vignetting.
Lenses used in wireless telephones (e.g., cellular telephones) present a unique challenge because they must be compact, inexpensive, used with a digital pixel array and provide a high-quality picture. To achieve these goals, designers have used wafer-level optics (WLO), which involves packaging small lenses with the digital circuitry, including a pixel array. Currently, however, lenses used in wafer-level optics are highly aspheric, or “free form,” often with aspheric coefficients of the 10th to 14th order, unlike lenses used in more traditional cameras.
FIG. 1 shows an example of a packaged lens system 1 used in a cell phone camera. Lens system 1 has an outer lens substrate 2 and an inner lens substrate 3, wherein inner lens substrate 3 is between outer lens substrate 2 and the image plane on a pixel array. Outer lens substrate 2 has one positive lens 7, and inner lens substrate 3 has two highly aspheric, or “free form” lenses 5, 6. Outer lens substrate 2 may have a second lens as well. Lens system 1 includes a spacer 8 that is connected to inner lens substrate 3 and outer lens substrate 2 and fixes the lenses in the position shown in FIG. 1.
In current wafer-level lens designs such as the illustrated lens system 1, there is a lack of symmetry and thus strong ray bending (e.g., ray bundle W) to achieve a short total track of the imaging systems with a comparatively large focal length and small chief ray angle (CRA). Aberrations introduced by these strict specifications are reduced by introducing free form lenses 5, 6 with high order aspheric coefficients. These free form lenses actually cause an increase in aberrations if, by the influence of fabrication tolerances, the ray bundles do not follow their expected path through the lenses. To reduce size, lens system 1 requires lens 6 be separated from the image plane by distance d, which also requires that lens 6 have a larger diameter than would be required if lens 6 were farther from the image plane. Moreover, fabrication and testing of free form structures can be complicated.
General design principles used in conventional camera lenses can remedy many of the issues experienced in current cell phone camera lens systems. One known implementation of some of the general design principles is the Cooke Triplet 20 shown in FIG. 2. The Cooke Triplet 20 is a configuration of an inner negative lens 23 and two outer positive lenses 21, 22. Due to the “triplet” configuration of the Cooke Triplet 20, requiring three individual lens structures 21, 22, 23, the Cooke Triplet 20 can be excessively large, thus increasing the cost of materials. Furthermore, fabrication at a wafer level can be expensive using conventional methods and materials.
General design principles used in conventional cameras are currently neglected in cell phone cameras. The application of these rules, however, requires maximizing system symmetry, meaning that the maximum chief ray angle (CRA) on the image sensor is sufficiently equal to the corresponding maximum field of view angle.
Accordingly, there is a need and a desire for a simple, small and inexpensive lens design that incorporates general optics design principles by maximizing system symmetry.